Three dimensional interpenetrating networks of macroscopic assemblages of randomly oriented carbon fibrils and organic polymers

ABSTRACT

The invention relates to an interpenetrating network of carbon fibrils and a polymer, which comprises a rigidized, three-dimensional macroscopic assemblage of a multiplicity of randomly oriented carbon fibrils and an innerpenetrating mass of an organic polymer. The invention also relates to methods of making and using such innerpenetrating networks.

FIELD OF THE INVENTION

The invention relates generally to interpenetrating networks of carbonfibrils and polymers. More specifically, the invention relates to aninterpenetrating network of carbon fibrils and a polymer, whichcomprises a rigidized, three dimensional macroscopic assemblage of amultiplicity of randomly oriented carbon fibrils and an interpenetratingmass of a polymer. Even more specifically, the invention relates tomethods for making such interpenetrating networks by forming porousrigidized structures and forming an organic polymer therein.

BACKGROUND OF THE INVENTION

Carbon fibrils are vermicular carbon deposits having diameters less than500 nanometers (nm). They exist in a variety of forms, and have beenprepared through the catalytic decomposition of variouscarbon-containing gases at metal surfaces.

Tennent, U.S. Pat. No. 4,663,230, describes carbon fibrils that are freeof a continuous thermal carbon overcoat and have multiple graphiticouter layers that are substantially parallel to the fibril axis. As suchthey may be characterized as having their c-axes, the axes which areperpendicular to the tangents of the curved layers of graphite,substantially perpendicular to their cylindrical axes. They generallyhave diameters no greater than 0.1 micron and length to diameter ratiosof at least 5. Desirably they are substantially free of a continuousthermal carbon overcoat, i.e., pyrolytically deposited carbon resultingfrom thermal cracking of the gas feed used to prepare them.

Fibrils are useful in a variety of applications. For example, they canbe used as reinforcements in fiber-reinforced network structures orhybrid network structures, i.e., networks containing reinforcements suchas continuous fibers in addition to fibrils.

In recent years, much interest has been expressed in the formation ofmolecular composites, i.e. composites in which individual molecules ofrigid rod polymers are dispersed in more flexible matrix polymers toform mutually interpenetrating networks. It is generally believed thatsuch composites will be able to withstand stresses substantially greaterthan conventional composites because stress will be distributedthroughout the interpenetrating molecular system. Also, such compositeswill be less likely to suffer localized stress and will be able towithstand higher stress and/or strain before failure.

U.S. patent application Ser. No. 08/057,328 filed May 5, 1993 [PCTUS94/04879, WO 94/25268], hereby incorporated by reference, disclosesmethods for forming three dimensional macroscopic assemblages ofrandomly oriented carbon fibrils. Broadly, dispersions of fibrils in acompatible liquid are prepared and the liquid is then removed to form alow-density porous plug or mat. In a preferred method, a low-densityporous fibril plug is prepared by dispersing the fibrils in solvent,e.g., n-pentane, the dispersion is charged to a pressure vessel, thevessel, is heated above the critical temperature of the solvent, and thesupercritical vapor is bled out of the vessel. In this manner a solidplug having the shape of the vessel interior is obtained.

U.S. patent application Ser. No. 08/857,383, filed May 15, 1997, herebyincorporated by reference, describes rigid, porous carbon structurescomprised of carbon fibrils. The fibrils are bonded or glued to oneanother at their intersections. Bonding can be induced by chemicalmodification of the surface of the fibers to promote bonding, by adding"gluing" agents and/or by pyrolyzing the fibrils to cause fusion orbonding at the points of interconnection.

OBJECTS OF THE INVENTION

It is a primary object of the invention to produce a molecular compositebased on a network of carbon fibrils interpenetrated with a polymer.

It is a further object of the invention to create an interpenetratingnetwork of carbon fibrils and an organic polymer which has theadvantages of molecular composites, i.e. superior stress resistance,resistance to cracking and toughness.

It is a further and related object of the invention to provide methodsfor producing rigidized, three-dimensional, macroscopic assemblages ofrandomly oriented carbon fibrils with an organic polymer.

It is still a further object of the invention to provide suchinterpenetrating networks of fibrils of different morphology, togetherwith a range of different polymer substances, and to produce suchnetworks in an efficient and cost effective manner.

It is still a further and related object of the invention to create therigidized structure by processes which best permit a subsequent, insitu, polymerization of the monomer within the porous, rigidized fibrilstructure.

SUMMARY OF THE INVENTION

These and other objects of the invention are achieved byinterpenetrating networks of carbon fibrils and polymer, comprising arigidized, three-dimensional, macroscopic assemblage of a multiplicityof randomly oriented carbon fibrils and an interpenetrating mass of apolymer. Such interpenetrating networks can be obtained by forming arigidized, three-dimensional, porous, macroscopic assemblage of amultiplicity of randomly oriented carbon fibrils, introducing a liquidor gas phase organic monomer into the porous interior of the assemblage,together with an appropriate catalyst or free-radical initiator, andcausing the monomer to polymerize under polymerization conditions withinthe assemblage.

DETAILED DESCRIPTION OF THE INVENTION Definitions

The term "fibril" refers to carbon fibers having a small diameter andincludes fibrils, whiskers, nanotubes, buckytubes, etc.

The term "assemblage" refers to any configuration of a mass ofindividual fibrils and embraces intertwined as well as discrete fibrilembodiments.

The term "macroscopic" means that the assemblages may be of any suitablesize to achieve an industrial or scientific purpose.

Fibrils

The fibrils used in the invention have a diameter less than 1000 nm,preferably less than about 200 nm, even more preferably less than 100 nmand most preferably less than 50 nm. According to one embodiment of theinvention, carbon fibrils having diameters in the range of 3.5 to 70 nmare used to form the rigid assemblage.

U.S. Pat. No. 4,663,230 to Tennent, hereby incorporated by reference,describes carbon fibrils that are free of a continuous thermal carbonovercoat and have multiple ordered graphitic outer layers that aresubstantially parallel to the fibril axis. As such they may becharacterized as having their c-axes, the axes which are perpendicularto the tangents of the curved layers of graphite, substantiallyperpendicular to their cylindrical axes. They generally have diametersno greater than 100 nm and length to diameter ratios of at least 5.Desirably they are substantially free of a continuous thermal carbonovercoat, i.e., pyrolytically deposited carbon resulting from thermalcracking of the gas feed used to prepare them. The Tennent inventionprovided access to smaller diameter fibrils, typically 3.5 to 70 nm andto an ordered, "as grown" graphitic surface. Fibrillar carbons of lessperfect structure, but also without a pyrolytic carbon outer layer havealso been grown.

U.S. Pat. No. 5,171,560 to Tennent et al., hereby incorporated byreference, describes carbon fibrils free of thermal overcoat and havinggraphitic layers substantially parallel to the fibril axes such that theprojection of said layers on said fibril axes extends for a distance ofat least two fibril diameters. Typically, such fibrils are substantiallycylindrical, graphitic nanotubes of substantially constant diameter andcomprise cylindrical graphitic sheets whose c-axes are substantiallyperpendicular to their cylindrical axis. They are substantially free ofpyrolytically deposited carbon, have a diameter less than 100 nm and alength to diameter ratio of greater than 5. These fibrils are of primaryinterest in the invention.

When the projection of the graphitic layers on the fibril axis extendsfor a distance of less than two fibril diameters, the carbon planes ofthe graphitic nanofiber, in cross section, take on a herring boneappearance. These are termed fishbone fibrils. Geus, U.S. Pat. No.4,855,091, hereby incorporated by reference, provides a procedure forpreparation of fishbone fibrils substantially free of a pyrolyticovercoat. These fibrils are also useful in the practice of theinvention.

Oxidized nanofibers may be used to form the rigid porous assemblage.McCarthy et al., U.S. patent application Ser. No. 351,967 filed May 15,1989, hereby incorporated by reference, describes processes foroxidizing the surface of carbon fibrils that include contacting thefibrils with an oxidizing agent that includes sulfuric acid (H₂ SO₄) andpotassium chlorate (KClO₃) under reaction conditions (e.g., time,temperature, and pressure) sufficient to oxidize the surface of thefibril. The fibrils oxidized according to the processes of McCarthy, etal. are non-uniformly oxidized, that is, the carbon atoms aresubstituted with a mixture of carboxyl, aldehyde, ketone, phenolic andother carbonyl groups. Fibrils have also been oxidized non-uniformly bytreatment with nitric acid. International Application PCT/US94/10168discloses the formation of oxidized fibrils containing a mixture offunctional groups.

In published work, McCarthy and Bening (Polymer Preprints ACS Div. ofPolymer Chem. 30 (1)420(1990)) prepared derivatives of oxidized fibrilsin order to demonstrate that the surface comprised a variety of oxidizedgroups. Fibrils may also be oxidized using hydrogen peroxide, chlorate,nitric acid and other suitable reagents.

The fibrils may be further functionalized as set forth in U.S. patentapplication Ser. No. 08/352,400, filed Dec. 8, 1995, by Hoch and Moy etal., entitled "Functionalized Fibrils", hereby incorporated byreference.

Carbon fibrils of a morphology similar to the catalytically grownfibrils described above have been grown in a high temperature carbon arc(Iijima, Nature 354 56 1991, hereby incorporated by reference). It isnow generally accepted (Weaver, Science 265 1994, hereby incorporated byreference) that these arc-grown fibrils have the same morphology as theearlier catalytically grown fibrils of Tennent. Arc grown carbon fibrilsare also useful in the invention.

The fibrils may also be high surface area fibrils disclosed in U.S.Provisional Application Ser. No. 60/017,787 (CMS Docket No.:370077-3630) entitled "High Surface Area Nanofibers, Methods of Making,Methods of Using and Products Containing Same", hereby incorporated byreference.

Fibril Aggregates and Assemblages

The "unbonded" precursor fibrils may be in the form of discrete fibers,aggregates of fibers or both. Aggregates, when present, are generally ofthe bird's nest, combed yarn or open net morphologies. The more"entangled" the aggregates are, the more processing will be required toachieve a suitable composition if a high porosity is desired. This meansthat the selection of combed yarn or open net aggregates is mostpreferable for the majority of applications. However, bird's nestaggregates will generally suffice.

The fibril mats or assemblages have been prepared by dispersing fibrilsin aqueous or organic media and then filtering them to form a mat orassemblage.

Rigidized assemblages are prepared by intimately mixing fibrils with aglue, e.g. sugar, glycerin, polyethylene oxide, polyethylene glycol,polyacrylamide or polyacrylic acid, or, with carbonizable resins, suchas phenolic resins, in a kneader, followed by extruding or pelletizingand pyrolyzing. Rigidized assemblages have also been prepared by forminga gel or paste of fibrils in a fluid, e.g. a solvent such as carbondioxide, acetone, or a C₂ -C₇ alkane or alkene, heating the gel or pasteto a temperature above the critical temperature of the medium, removingsupercritical fluid and finally removing the resultant porous mat orplug from the vessel in which the process has been carried out. See,U.S. patent application Ser. No. 08/057,328 entitled Three-DimensionalMacroscopic Assemblages of Randomly Oriented Carbon Fibrils andComposites Containing Same and U.S. patent application Ser. No.08/857,383 entitled Rigid Porous Carbon Structures, Methods of Making,Methods of Using and Products Containing Same referred to above.

Polymerization of Monomers in Rigid Assemblages

An interpenetrating network of the rigidized, three-dimensionalmacroscopic assemblage of randomly oriented fibrils and a polymer can beformed by introducing a monomer into the rigidized assemblage togetherwith a suitable polymerization catalyst and causing polymerization totake place under suitable conditions. The monomer may be in the liquidor the gas phase. Suitable monomers include vinyl compounds, i.e.compounds having a terminal double bond, e.g. styrene, substitutedstyrene, methyl methacrylate, alpha olefins, substituted alpha olefins,etc.

The monomer can be any compound that can be polymerized by either freeradical mechanisms or by Ziegler-Natta catalysis. The catalyst locationand the polymerization rate must be adjusted by those skilled in the artto avoid having most of the polymer formed on the outside edges of thefibril network. In such event, the interior of the porous assemblagewill have void spaces and the composition will be nonuniform andtherefore unsatisfactory. Accordingly it is preferred to evenly andthoroughly distribute the catalyst throughout the interior of theassemblage.

Fibrils have free radical traps and oxidized fibrils have even more freeradical traps and accordingly it is advantageous to treat the rigidizedassemblages in order to reduce the number of those free radical traps,particularly where the mechanism of the contemplated polymerization isbased on free radicals. Such mechanisms are advantageous in the methodsof the invention because initiators can be distributed throughout thefibril network before monomer is introduced and polymerization can betriggered uniformly by a temperature change.

Ziegler-Natta catalysts such as those derived from TiCl₄ and ZrCl2(Cp)₂can also be adsorbed on the fibrils and thus can uniformly catalyze apolymerization reaction, e.g. of polypropylene, throughout theassemblage. Styrene can be polymerized, in situ, by both free radicalinitiation and Ziegler catalysis. Styrene is a suitable monomer becauseof its low cost and because it typically forms brittle matrices that canbe toughened with carbon fibrils.

Propylene, the lowest cost monomer, can be polymerized with a Zieglercatalyst distributed in the rigidized assemblage, and can be polymerizedto isotactic (crystalline) polypropylene, useful for its higher meltingpoint, hardness, stiffness and toughness relative to other polyolefins.

EXAMPLES EXAMPLE I Preparation of Carboxylic Acid-Functionalized FibrilsUsing Nitric Acid

A weighed sample of fibrils was slurried with nitric acid of theappropriate strength in a bound bottom multi-neck indented reactor flaskequipped with an overhead stirrer and a water condenser. With constantstirring, the temperature was adjusted and the reaction carried out forthe specified time. Brown fumes were liberated shortly after thetemperature exceeded 70° C., regardless of acid strength. After thereaction, the slurry was poured onto cracked ice and diluted with DIwater. The slurry was filtered and excess acid removed by washing in aSoxhlet extractor, replacing the reservoir with fresh DI water everyseveral hours, until a slurried sample gave no change in pH from DIwater. The fibrils were dried at 100° C. at 5" vacuum overnight. Aweighed portion of fibrils was reacted with standard 0.100 N NaOH andthe carboxylic acid content determined by back-titration with 0.100 NHCl. Surface oxygen content was determined by XPS. Dispersibility inwater was tested at 0.1 wt % by mixing in a Waring Blender at high for 2min. Results are summarized in the Table I below.

                                      TABLE 1                                     __________________________________________________________________________    Summary of Direct Oxidation Runs                                              Disp.                                                                            COMPONENTS                                                                 Ex.                                                                              Gms.                                                                              cc  Acid                                                                              Temp.   Wgt.                                                                              COOH                                                                              ESCA,                                                                              at %                                        H.sub.2 O Fibrils  Acid  Conc.  ° C.    Time   Loss   meg/g                                                   C      O                             __________________________________________________________________________    12A                                                                               1(BN)                                                                            300 70% RT  24 hr                                                                             0   <0.1                                                                              98   2                                           P                                                                           12B                                                                               1(BN)                                                                            300 70  rflx                                                                              48  <5% <0.1                                                                              not                                              P                     analyzed                                                12C     20(BN)   1.01  70     rflx    7    25%    0.8      not                G                     analyzed                                                12D     48(BN)   1.01  70     rflx   7     20%    0.9      not                G                     analyzed                                              __________________________________________________________________________     P = Poor;                                                                     G = Good                                                                 

EXAMPLE II Preparation of a Rigidized Low-Density Porous Fibril Plug

Supercritical fluid removal from a well dispersed-fibril paste is usedto prepare low density shapes. 50 cc of a 0.5% dispersion of the fibrilsfrom Example 1 above in n-pentane is charged to a pressure vessel ofslightly larger capacity which is equipped with a needle valve to enableslow release of pressure. After the vessel is heated above the criticaltemperature of pentane (Tc=196.6°), the needle valve is cracked openslightly to bleed the supercritical pentane over a period of about anhour.

The resultant solid plug of Fibrils, which has the shape of the vesselinterior, has a density of 0.005 g/cc, corresponding to a pore volumefraction of 0.997%. The resistivity is isotropic and about 20 ohm/cm.

The preform is rigidized by heating to 650° C. in argon for 1 hr.Alternatively the preform is rigidized by heating to 300° C. in air forone hour.

EXAMPLE III Preparation of a Rigidized Low-Density Porous Fibril Plug

0.333 g resorcinol, (Aldrich) is dissolved in 5.3 cc H₂ O. After 0.491 gformaldehyde solution (37% in H₂ O, Aldrich) is added, the solution ismixed thoroughly with 8.879 g fibril slurry (5.8%). After the additionof 7.4 cc 0.2M Na₂ CO₃, the mixture is transferred to a glass vial. Thesealed vial is placed in an oven at 80° C. for four days. The gel formedis washed with water. Finally, the water in the gel is exchanged withacetone.

After the supercritical acetone in the gel is removed, the product maybe heated under Ar at 400° C. for 2 hr., 800° C. for 4 hr. and 1200° C.for 4 hr. to carbonize the resoreinol-formaldehyde polymer.

EXAMPLE IV Preparation of a Rigidized Low-Density Porous Fibril Plug

Supercritical fluid removal from a well dispersed-fibril paste is usedto prepare low density shapes. 50 cc of a 0.5% dispersion of "as made"fibrils in n-pentane containing a mixture of phenolformalydehyde/PEG/Glycerin as a glue is charged to a pressure vessel ofslightly larger capacity which is equipped with a needle valve to enableslow release of pressure. After the vessel is heated above the criticaltemperature of pentane (Tc=196.6°), the needle valve is cracked openslightly to bleed the supercritical pentane over a period of about anhour.

The resultant solid plug of fibrils, which has the shape of the vesselinterior is air dried and heated to 350° C. to remove PEG/Glycerin andpyrolyze the phenolic resin. The plug has a density of 0.005 g/cc,corresponding to a pore volume fraction of 0.997%.

EXAMPLE V

Porous preforms prepared as described in Examples II, III and IV arehydrogenated at 800-1400° C. in an aluminum tube using 99.999% (highpurity) hydrogen and cooled to room temperature under argon.

The preforms are infiltrated with a mixture of styrene monomer/benzylperoxide. The preform is then heated to 80° for 1 hr. to polymerize thestyrene monomer. A solid polystyrene/fibril network is obtained.

EXAMPLE VI

Preforms prepared as in Examples II, III or IV are placed in aserum-capped pressure bottle containing a magnetic stirring bar. Thebottle is evacuated and filled with dry nitrogen several times. It isthen chilled to 0° C. and sparged with nitrogen which passes through aserum-capped bottle containing ca. 0.01 gm TiCl₄ per gm of preform at50° C. until all the TiCl₄ has evaporated. The bottle containing thefibrils was then held at 50° C. until all the TiCl₄ has evaporated. Thebottle containing the fibrils is then held at 50° C. for an hour todistribute the TiCl₄ on the fibrils. Triethyl aluminum in the amount of0.03 gm per gm of preform is similarly introduced.

With the bottle at 25° C., a stream of ethylene is introduced atatmospheric pressure. A strong exotherm indicates that the ethylene ispolymerizing. The black fibrils turn grey, then nearly white.Polymerization is continued until ca. 150 gm per gm of preform ofpolyethylene has formed. An interpenetrating network of fibrils andpolyethylene is obtained.

EXAMPLE VII

A mixture of 1 m mol methyl alomoxane and 10 μmol biscyclopenta dienylzirconium dichloride in benzene is prepared and added to a rigidizedpreform prepared according to Example II, III or IV to the point ofincipient wetness. The system is cooled to below the freezing point ofbenzene and the benzene is removed by sublimation. The treated preformis taken to dry ice temperature and infiltrated with liquid propylene at100 psi. The system is slowly warmed until polymerization initiates andmaintained at that temperature for one hour. An interpenetrating networkof fibrils and polypropylene is obtained.

What is claimed is:
 1. An interpenetrating network of carbon fibrils anda polymer comprising:(a) a rigidized, three dimensional macroscopicassemblage of a multiplicity of randomly oriented carbon fibrils, and(b) an interpenetrating mass of a polymer infiltrated within saidassemblage.
 2. An interpenetrating network of carbon fibrils and apolymer comprising:(a) a rigidized, three-dimensional, macroscopicassemblage of a multiplicity of randomly oriented carbon fibrils, saidfibrils being substantially cylindrical with a substantially constantdiameter, having c-axes substantially perpendicular to their cylindricalaxis, being substantially free of pyrolytically deposited carbon andhaving a diameter between about 3.5 and 70 nanometers, and (b) aninterpenetrating mass of an organic polymer infiltrated within saidassemblage.
 3. An interpenetrating network of carbon fibrils and apolymer produced by a method comprising the steps of:(a) forming arigidized, three dimensional, porous, macroscopic assemblage of amultiplicity of randomly oriented carbon fibrils; (b) introducing aliquid or gas phase monomer into the interior of said assemblagetogether with a polymerization catalyst or initiator; and (c) causingsaid monomer to polymerize within said assemblage thereby forming saidinterpenetrating network of carbon fibrils and polymer, wherein thepolymer infiltrated with said assemblage.
 4. An interpenetrating networkof carbon fibrils and a polymer produced by a method comprising thesteps of:(a) forming a rigidized, three-dimensional, porous, macroscopicassemblage of a multiplicity of randomly oriented carbon fibrils, saidfibrils being substantially cylindrical with a substantially constantdiameter, having c-axes substantially perpendicular to their cylindricalaxis, being substantially free of pyrolytically deposited carbon andhaving a diameter between about 3.5 and 70 nanometers; (b) introducing aliquid or gas phase organic monomer into the porous interior of saidassemblage together with a Ziegler catalyst or an initiator; and (c)causing said monomer to polymerize within said assemblage therebyforming said interpenetrating network of carbon fibrils and polymer,wherein the polymer infiltrated within said assemblage.
 5. Theinterpenetrating network of claim 1, wherein said carbon fibrils have anaverage diameter less than 1000 nm.
 6. The interpenetrating network ofclaim 1, wherein said carbon fibrils have an average diameter less than200 nm.
 7. The interpenetrating network of claim 1, wherein said carbonfibrils have an average diameter less than 100 nm.
 8. Theinterpenetrating network of claim 1, wherein said carbon fibrils havediameters in the range of 3.5 to 70 nm.
 9. The interpenetrating networkof claim 1, wherein said carbon fibrils are free of a continuous thermalcarbon overcoat.
 10. The interpenetrating network of claim 1, whereinsaid rigidized, three dimensional, macroscopic assemblage is prepared bydispersing carbon fibrils an aqueous or organic media and then removingthe media to form said assemblage.
 11. The interpenetrating network ofclaim 1, wherein said rigidized, three dimensional, macroscopicassemblage is prepared by intimately mixing fibrils with a glue to formsaid rigidized assemblage.
 12. The interpenetrating network of claim 1,wherein said interpenetrating mass of polymer is prepared by introducinga monomer into the rigidized assemblage together with a suitablepolymerization catalyst or initiator and causing the polymerization totake place within the assemblage.
 13. The interpenetrating network ofclaim 1, wherein said polymer comprises monomers selected from the groupconsisting of styrene, substituted styrene, methyl methacrylate, alphaolefins, substituted alpha olefins or mixtures thereof.
 14. Theinterpenetrating network of claim 1, wherein said interpenetratingnetwork is substantially free of void spaces.
 15. The interpenetratingnetwork of claim 12, wherein said monomer is a gas.
 16. Theinterpenetrating network of claim 12, wherein said monomer is a liquid.17. The interpenetrating network of claim 1, wherein said rigidized,three dimensional, porous, macroscopic assemblage is formed fromoxidized carbon fibrils.
 18. The interpenetrating network of claim 3,wherein step (c) comprises evenly and thoroughly distributing saidpolymerization catalyst or initiator throughout the interior of theassemblage.